OFFSET
1,3
COMMENTS
The number of free polyominoes of size 4n that have 90-degree rotational symmetry about a point that coincides with the corner of a square, and that have not at the same time reflective symmetry. Note that for polyominoes which have a hole in the center, the center of rotation will be the corner of a square within the hole, rather than being the corner of a square of the polyomino itself. The sequence is defined for 4n rather than n as polyominoes of size not a multiple of 4 cannot have the required symmetry.
The sequence enumerates a subset of the polyominoes enumerated by A144553.
CROSSREFS
KEYWORD
nonn,more
AUTHOR
John Mason, Dec 18 2013
EXTENSIONS
a(8)-a(13) from Sean A. Irvine, Jul 04 2019
a(14)-a(18) from John Mason, Feb 02 2022
STATUS
approved